Fuel cells can be used for the environmentally friendly generation of electricity. In a fuel cell, a process which substantially represents a reversal of electrolysis takes place. For this purpose, in a fuel cell, a fuel which includes hydrogen is fed to an anode. Further, an auxiliary substance which includes oxygen is fed to a cathode. The anode and cathode are electrically separated from one another by an electrolyte layer.
Although the electrolyte layer allows ion exchange between the fuel and the oxygen, it otherwise ensures gastight separation of the fuel and auxiliary substance. As a result of the ion exchange, hydrogen which is present in the fuel can react with the oxygen to form water, with electrons accumulating at the fuel-side electrode or anode and electrons being taken up at the electrode on the side of the auxiliary substance, or the cathode.
Therefore, when the fuel cell is operating, a usable potential difference or voltage builds up between anode and cathode, while the only waste product of the electricity generation process is water. The electrolyte layer, which in the case of a high-temperature fuel cell may be formed as a solid ceramic electrolyte or in the case of a low-temperature fuel cell may be formed as a polymer membrane, therefore has the function of separating the reactants from one another, transferring the charge in the form of ions and preventing an electron short circuit.
A fuel cell comprises a flat electrolyte, one flat side of which is adjoined by a flat anode and the other flat side of which is adjoined by a cathode, which is likewise flat. These two electrodes, together with the electrolyte, form what is known as an electrolyte electrode assembly. Adjacent to the anode there is an anode gas space, and adjacent to the cathode there is a cathode gas space.
An interconnector plate is arranged between the anode gas space of a fuel cell and the cathode gas space of a fuel cell adjacent to this fuel cell. The interconnector plate produces an electrical connection between the anode of the first fuel cell and the cathode of the second fuel cell. Depending on the type of fuel cell, the interconnector plate is designed, for example, as a single metallic plate or as a cooling element which comprises two plates stacked on top of one another with a cooling-water space between them. Depending on the type of fuel cell, there are further components, such as for example electrically conductive layers, seals or pressure cushions, in a fuel cell stack.
On account of the electrochemical potentials of the materials which are customarily used, in a fuel cell of this type an electrode voltage of approximately 0.6 to 1.0 V can be built up under normal operating conditions and maintained during operation. For industrial applications, in which a significantly higher total voltage may be required, depending on the intended use or the planned load, therefore, it is customary for a plurality of fuel cells to be electrically connected in series in the form of a fuel cell stack. The fuel cells are stacked in such a manner that the sum of the electrode voltages supplied by the fuel cells corresponds to the required total voltage or exceeds this total voltage. Depending on the total voltage required, the number of fuel cells in a fuel cell stack of this type may, for example, amount to 50 or more.
To make it possible to utilize the potential difference generated when the fuel cells which have been connected up to form a fuel cell stack of this type are operating, the fuel cell stack is connected to a load. In this case, for electrical connection of the load to the fuel cell stack, there is what is known as a terminal plate, to which electrical supply and discharge lines can be connected, arranged at the two outermost fuel cells of the fuel cells which are connected in series.
On account of the particular operating properties of fuel cells of this type, and in particular since water is the only significant by-product produced, fuel cells are also particularly suitable for use for supplying energy in closed mobile systems, such as for example underwater vessels. In this context, it is particularly advantageous that a relatively high output current can be achieved at a standard voltage level in the form of a relatively high power density in a fuel cell arrangement with only limited spatial dimensions. Moreover, particularly for use in underwater vessels, the fuel, i.e. the substance which includes hydrogen, can be provided in relatively compact form. The auxiliary substance or oxidizing agent used may in this case be pure oxygen. The hydrogen may in this case in particular be carried along in hydride tanks.
Particularly when fuel cells are used in an underwater vessel, it may be desirable for the signature emitted to the outside, i.e. the externally detectable signs indicating the location or operation of the underwater vessel, to be kept at a particularly low level. This signature may also include magnetic fields which are generated by the currents flowing in and out when fuel cells are operating.